To date, an inspection technique using a direct current potential difference method which measures a potential difference distribution on a specimen surface while sending a direct current through the specimen, which is an inspection subject structure, and indirectly detects a damage, such as an existence or otherwise of a crack in the specimen and a condition of the crack, from the potential difference distribution, has been known as one nondestructive inspection technique.
Recently, a nondestructive inspection method has also been proposed which, using the direct current potential difference method, enables a quantification of a damage such as a crack buried inside a specimen, or a crack exposed on a surface, as a three-dimensionally shaped damage having an optional aspect ratio and inclination.
That is, by first sending a predetermined current through the specimen and acquiring potential difference values at predetermined positions, and next, using analysis means such as a finite element method, setting a shape of a damage such as a hypothetical crack, which is to be a subject of detection, and carrying out a calculation based on the acquired potential difference values, the method identifies a shape of the damage (for example, refer to Patent Document 1 and Patent Document 2).    Patent Document 1: JP-A-64-035357    Patent Document 2: JP-A-06-109684